MATERIALS LABORATORY
DEPARTMENT OF CHEMICAL ENGINEERING
CRAWFORD HALL
UNIVERSITY OF
COPYRIGHT - Richard Brown, July 1982. ALL RIGHTS RESERVED
Contents
Introduction
Report Writing
Crystallography - Week 1
FCC, BCC and HCP Structures - Week 2
Metallography -Week 3
Mechanical Properties of Materials - Week 4
Cold Working - Week 5
Recrystallisation- Week 6
Presentation - Week 7
Age Hardening - Week 8
Heat Treatment of Steels - Week 9
Failure Analysis - Week 10
Corrosion - Week 11
Examination of a Pump - Week 12
Presentation - Week 13.
Introduction
In
this handbook is all the necessary information for completion of the Laboratory
sections of the Materials Engineering courses.
The purpose, theory, objectives and techniques of the experiments are
detailed as well as the necessary components of a laboratory report. Correctly used, these laboratories will
supplement and expand upon the theory gained in lectures.
To
maximize the information obtained from these laboratories, read the relevant
experimental section before attending the class. The instructor will expect you to have done
this, and the laboratory will be taught with this presumption. If you have any difficulties understanding
the notes, do not hesitate to ask questions of the instructor. Remember, the aim of these laboratories is
for you to gain a practical understanding of materials, develop a
reporting technique so other people may understand your research, and also
obtain “hands-on” experience of modern laboratory techniques and equipment.
Report Writing
An
undergraduate experimental course is designed to fulfil at least three
important requirements:
(1)
to provide experience in the
manipulation of equipment.
(2) to confirm practically he information
obtained from books and lectures.
(3) to give practice in the critical
presentation of the work in written form.
The
importance of the third requirement cannot be overstated. A scientific report is the method by which
experimental facts, together with the author’s analyses and opinions, are most
effectively transmitted. It should be
noted that the report may be the only permanent record of the work. Judgement of the quality of your work by
peers at a later date may have to be made from a review of your reports. Good experimental work therefore deserves a
well-presented report, otherwise a considerable amount of its value may be
lost.
It
will be seen from an inspection of scientific journals that, firstly the styles
of the various authors differ, and secondly the order of presentation in a
report usually conforms to the following sequence, even though the
precise division of the work may not be used.
*(1) Introduction - to explain in brief the
aim and purpose of the work.
(2) Previous
work.
(3) Theory
known before the experimental is performed.
(4) Equipment
and materials.
*(5) Experimental procedure.
*(6) Experimental results.
*(7) Discussion - including any theory
established as a result of the experiment.
*(8) Conclusions and recommendations - if
any.
*(9) References - to other literature and to
your own previously written accounts.
(10) Appendices.
In some papers it has been found
convenient to include details about previous work and theory in the
introduction, while in others the equipment and materials used have been
described adequately in the section on experimental procedure. It is suggested that student reports be
divided and sections be added as occasion demands. References to other literature should be
included whenever possible so that they may be used to retrace the appropriate
information at a later date. References
may also indicate experimental procedure described in earlier accounts so as to
avoid unnecessary duplication.
Appendices are rarely needed in brief reports and should only be used
when the information they contain cannot simply be included in the body of the
report. Abbreviations should be avoided
unless they conform to standard practice.
A
summary prior to the introduction usually accompanies published work but it is
not really part of the paper. It should
in fact be a simple precis of the whole of the report (procedure, results and
discussion) and should be complete and intelligible by itself.
Reports
are by convention written in an impersonal manner, and in describing work
accomplished, the past tense should be used.
The present tense is, however, preferred to describe still-existing
equipment or to outline current practice, e.g. the heat-treatments used in
industry.
The
experimental procedure should be included in detail sufficient to facilitate
repetition by another worker. No
relevant information should deliberately be aimed from the report; any mistakes
made in the experimental procedure should be reported and an assessment made of
their possible influence.
Results
should, where appropriate, be tabulated clearly. The ultimate aim should at all times be that
of making results and conclusions readily and rapidly available to the
reader, who should not need the entire text for this information.
Graphs
should be titled clearly, and experimental points should each be enclosed in a
circle so that the precise reading is not obscured by a line. Where an error has been estimated, the fact
should be indicated by a vertical line of appropriate length, extending above
and below the reading; in the case of errors being estimated for both
variables, this should be indicated by an upright cross of the appropriate size
centered at the reading.
A
critical discussion of experimental results is most important. It should relate your results to previous
work of a similar nature. A comparison
with theory should also be made and a discussion of the applicability of such
theories carried out. For quantitative
work, an assessment of the source and extent of error should be included. While maximum accuracy is the aim, it is a
mistake to claim excessive accuracy.
One
cannot over-emphasize the dangers of losing experimental data recorded on odd
scraps of paper. Results should
immediately be recorded in your laboratory notebook, and retained there even
after they have been included in the final report. All reports should be word processed. A separate title page should include the
title of the experiment, the names of the group members and name of the person
submitting the report. Individual
sections should be clearly titled and including those above marked with an
asterisk for a complete, representative report.
Neatness and tidiness of reports are important. An illegible report will not transmit any
information to the reader. A laboratory
report should be clearly presented.
Oral presentations are a very important
medium for providing information. Two presentations are scheduled during the
course, using Powerpoint software, laptop computers and video projector. The
aim of these it to give experience in oral presentations. Modern software makes
it very easy to produce a spectacular presentation that is itself the object of
interest rather than the information that should be conveyed to the audience Do
not make presentations that are overly graphical, keep them simple and to the
point.
Crystallography
Many
materials in nature occur as crystals.
Examples include the metallic elements gold, copper and silver; ionic
compounds such as salt (e.s. NaCl); ceramics, rutile Ti02; and
non-metallic elements, possibly the best known being carbon in the form of
diamonds. The science of the crystalline
state is called Crystallography.
A
crystal is best defined as having a periodic structure. That is a crystal is the repetition of a
specific arrangement of some known feature.
From this comes the concept of a “lattice”. A lattice is a framework built-up from
repetition in three dimensions of a series of lattice points. The property of a lattice point is that each
lattice point has identical surroundings. It is the feature which when repeated a large
number of times creates a crystal.
Figure 1 shows a lattice. The
dots at line intersections represent lattice points. Only 14 different combination of lattice
points are possible such that the lattice itself is not repeated. These 14 lattices are known as the Bravais
lattices and are shown in Fig. 2. They
are also known as “translation” groups.
Translation vectors a,b,c, (Fig. 1) describe the lattice group and serve
as reference axes.
In
many crystals, lattice points consist of groups of atoms. thus although only 14
lattices exist many different crystals structures exist, depending upon the
exact atomic arrangement of atoms at lattice points. However, for common metals and some other
elements of the periodic table single atoms occupy a lattice point.
To
specify a given point in a a lattice or
atom in a structure, a series of crystal axes or system of axes is used to
provide coordinates. The most common
metallic crystals are cubic and have a 3 axis system each at right angles. To describe the 14 Bravais lattices, only 7
systems of axes are used. Each has
specific equalities and inequalities in length and angels of the vectors
a,b,c. These are shown in table 1, where
a,b,c, are vectors and " $ and ( are angles. The angle opposite
is " etc. by convention. The axes a,b,c, are given values in units of
A and angles ",$,( are specified for crystal structures, see
Table 1.
Unit Cells
Coordinates of Position in a Unit Cell
A
position in a unit cell is specified from its coordinates. Let the point x,y,z in the unit cell have a
vector from the origin of rxyz = xa + yb+zc.
Its coordinates are therefore x,y,z.
Coordinates are expressed in terms of the length of cell edges, not
units of distances such as centimeters.
Thus, the 2,2,1, position is reached by moving a distance twice the unit
cell vector along the a axis, twice the vector b along the b axis and the
distance of unit cell vector c along the c axis. The point will be at a cell corner. For distances part way along cell edges or in
cell faces the coordinates will consist of fractions eg. ˝, ˝, ˝, is the center
of a cube.
Induces of lattice directions and crystal
planes
(a) Directions
in a
Directions
in a crystal are specified by using square parentheses, ie (uvw). This indicates the direction of a line from
the origin to a point, with coordinates u,v,w.
Convention sates that square brackets are used for a specific
direction, and only integers are used inside parentheses. These integers must be the smallest which
describe the direction. For example to
the point 3,3,3, the direction could be (333).
However (111) is conventially used, as it is parallel to (333). To prove this draw out three unit cells and
mark (333) and (111). To find a
direction in a crystal, draw a line through the origin parallel to the
direction required, Fig. 3. Give the coordinates
of a point on it, measured in cell edge lengths and convert to the smallest
integers, having the same ration, written as (uvw). For example, in Fig. 3a, the coordinates are
1/2a, 1b, 1/2c. The direction is written
(121).
Negative
indices are written with a bar over them.
For example, the coordinates of the position marked in Fig. 3b are 1a,
1b, 1c. Its direction is therefore
(111).
Symmetry
in a crystal makes directions with the same values of (uvw) identical. For example (100) is a direction along the a
axis, (010) along b, and (001) along c.
Such a family is shown by arrowed parentheses; (100)
represents all possibilities of (100), (010), (001), (T00), (0T0), (00T). There are six of these and they correspond to
the edges of a cube for a unit cell in the cubic system. (b) Planes in a crystal.
For
crystal planes a similar system exists, called Miller indices. They represent the orientation of a crystal
plane in a unit cell without respect o the origin, but with respect to the
crystal axes. Miller indices only
apply to three axes systems, and are based upon their intercepts with
crystal axes. The unit of measurement is
again the unit cell dimension, either a,b, or c. The intercepts of the plane on the three axes
from the origin are counted. Reciprocals
are then taken. These are then reduced
to integers having the same ratios.
Finally, these are enclosed in round parentheses ie (hkl). For example in Fig. 4, the shaded plane makes
intercepts of a, 1/2b, 1/3c. Reciprocals
produce a,2b,3c. Integers having the
same ratio are 1/1, 2/1, 3/1, therefore the plane is a (123) plane. The four commonest planes for cubic crystals
are shown in Fig. 5.
Curly
parenthesis {hkl} signify planes of similar form. For example:
{100} = (100) +(010) +(001) + (T00) + (0T0) +
(00T) for a single crystal type, planes of a form all have the same atomic
configuration. Thus for {100} there are
3 crystal equivalents. For cubic
crystals, the {100} family are the cube faces.
The
three axis system for the hexagonal system does not produce equivalent indices
for equivalent atomic planes. Thus a
four system notation is used, called Miller-Bravais indices. Four axes a1,a2,a3
and c are used, Fig. 6. Planes are
expressed as (hkil) and by convention h+k+I = 0.
Directions
are found using the geometrical approach shown in Fig. 6. Thus directions, for example along the a1
axis, are given by the translation along the dotted path, for
[2TT0]. For planes the intercepts along
a1,a2,a3 and c are used, again using cell edges as distance units reciprocals
taken and reduced to the lowest integer and placed in round parentheses. For example the plane cutting the c axis at a
distance c from the origin, but not intercepting a1,a2, or a3, has intercepts ", ", ", 1, reciprocals 0, 0, 0, 1. The lowest integer with same ratio is 0, 0,
0, 1, and the plane (0001) is called the “basal” plane. The hexagonal faces are of the family {10T0}. These are termed “prism planes”. The common hexagonal planes are shown in Fig.
7.
Manipulation of Indices
It
should be noted that in both the three and four coordinate systems, planes are
at 90o to directions of the same integers. For example a (111) is at 90o to
(111) and (0001) is at 90o [001]. This
leads to several useful properties, particularly in fields such as electron
microscopy.
Symmetry of
These
are shown below:
Crystal
Type Symmetry axes
Triclinic None
Monoclinic 1 two-fold
Orthorhombic 3 perpendicular two-fold
axes
Tetragonal 1 four-fold axis
Rhombohedral 1 three-fold axis
Hexagonal 1 six-fold axis
Cubic 4
three-fold axis
To be in a specific crystal class a structure
must have the required minimum rotational symmetry.
Experimental
A
worksheet will be provided in the lab.
Reference
The
structure of Metals by C.S. Barrett and T. B.
Masselki.
FCC, BCC and HCP Metals
Introduction
The majority of common metals have either a
Face Centre Cubic Structure, fig. 1a, a Body Centered Cubic Structure, fig. 1b
or an Hexagonal Close Packed structure fig. 1c.
These are usually abbreviated to FCC, BCC or HCP structures
respectively. The major differences
between these structures is the Unit Cell, the building block. These are shown in fig. 1. the different cells leads to different
physical properties of bulk metals. For
example, FCC metals, Cu, Au, Ag are usually soft and ‘ductile’, which means
they can be bent and shaped easily. BCC
metals are less ductile but stronger, eg iron, while HCP metals are usually
brittle. Zinc is HCP and is difficult to
bend without breaking, unlike copper.
Many other features depend upon the crystal structure of metals, such as
density, deformation processes, alloying behavior, and much more. Thus, it is important to fully understand
metal structures.
Face Centre Cubic Structure